This invention relates to an electrolyzing method in which a novel cation-exchange membrane is used and electrolysis is carried out by supplying an aqueous solution of an alkali metal halide into an anode chamber and water into a cathode chamber to obtain halogen from the anode chamber and hydrogen and alkali hydroxide from the cathode chamber and more particularly to a method for obtaining a high purity alkali hydroxide at a high rate of decomposition of halide of alkali metal and a high current efficiency through an electrolyzing process carried out with a cation-exchange membrane, which is prepared by arranging one side of a cation-exchange group containing fluorocarbon polymer membrane to have lower concentration of the cation-exchange group than that of the other side thereof within a range of depth from 1 to 100.mu. and by arranging this side of the membrane which has the lower exchange group concentration to face the anode chamber, which has an alkali metal halide supplied thereto.
Among the conventional diaphragms designed for use in electrolysis of alkali metal halides, there have appeared cation-exchange membranes of a class having a sulfonic acid group as exchange group with a fluorine-containing resin employed as substratum. Typical examples of the cation-exchange membranes of this class include a sulfonic acid type membrane made of a perfluorocarbon polymer which was marketed by Du Pont Co. However, although the membrane is impeccable in terms of durability, the rate of cation transport thereof in an electrolytic solution has not been satisfactory.
Hence, various methods for improvement in this respect have been studied and are still under studies. The following are examples of such methods for improvement:
(1) A method in which the concentration of the exchange group on other side of such a membrane facing a cathode chamber is arranged to be lower than that of the other side facting the anode chamber.
(2) A method in which the exchange group on the side faceing the cathode chamber is arranged to be more weakly acidic than that of the other side facing the anode chamber.
(3) Another method in which a weakly acidic exchange group is employed.
It is well known that the cost of production generally not only varies with power consumption but also greatly varies with the rate of decomposition of the alkali metal halide employed and the concentration of the alkali hydroxide produced. Further, a manufacturing method that enables to produce an alkali hydroxide at a low production cost would be hardly acceptable for industrial purposes if the purity of the alkali hydroxide produced is low. In order to have industrial manufacture efficiently carried out, the balance between the cost of production and the quality of the product must be thoroughly taken into consideration in developing an improved membrane or diaphragm.
With the above stated improved cation-exchange membranes of the prior art employed in carrying out electrolysis to obtain a high purity alkali hydroxide by enhancing the rate of decomposition of the alkali metal halide, however, it has often been experienced that the alkali hydroxide thus obtained has an alkali metal halide mixed therein as well as that the current efficiency is low in the production process.
The present invention has resulted from studies which have been strenuously conducted for the solution of the above stated problems of the prior art.